Hub unit for a high temperature electronic monitoring system

ABSTRACT

A hub unit adapted for use in a monitoring system that monitors engine performance parameters of a gas turbine engine. The hub unit includes a housing, at least one signal conditioning circuit board within the housing and adapted to receive the analog sensor outputs from the sensors, and a control circuit board within the housing, connected to the signal conditioning circuit board, and adapted to produce digital data corresponding to analog sensor outputs. The control circuit board and the signal conditioning circuit board each comprise electrical circuit components that define an analog signal processing path and have accuracy and precision characteristics that drift in response to component aging and to changes in the temperature to which the hub unit is subjected. The hub unit performs a continuous calibration scheme to determine and remove errors in the analog signal processing path resulting from the drifts of the electrical circuit components.

BACKGROUND OF THE INVENTION

The present invention generally relates to electronic equipment, andmore particularly to electronic hardware capable of operating within ahigh temperature environment, such as on or adjacent a gas turbineengine.

Aircraft gas turbine engines undergo testing during their development,as well as during production and subsequent servicing. Numerous engineperformance parameters are typically monitored to assess the performanceof an engine, including various temperatures, pressures, flow rates,forces, rotational speeds, etc. As nonlimiting examples, it is typicallydesirable to monitor engine inlet, compressor and exhaust gastemperatures, pressures within the fan, compressor and turbine sections,fuel and airflow rates, compressor and fan rotor speeds, blade tipclearances, mechanical stresses and part vibrations. Development andflight test aircraft engines may be required to have thousands ofsensors to monitor the various parameters of interest.

Engine testing is typically conducted on a stationary test stand that isoften located outdoors. A nonlimiting example of such a test stand 100is schematically represented in FIG. 1. The stand 100 is represented asincluding a vertical support column 102 mounted to a foundation 104 inthe ground, and a head (thrust) frame 106 mounted on the column 102 fromwhich an aircraft engine 108 is mounted for testing. The head frame 106includes an adapter 110 to which the engine 108 is attached with a pylon112 that is appropriately configured for the particular engine 108.

During engine testing, the engine 108 and its immediate surroundings canreach very high temperatures. For example, temperatures may approach orexceed 260° C. surrounding the engine core beneath the engine cowling(nacelle) 114, as well as on the head frame 106 and its adapter 110.While sensors used to monitor the engine 108 have been developed towithstand these temperatures, the electronics used to process the sensordata have been limited to much lower temperatures. For example, typicalcommercial electronic components are often limited to about 85° C., andeven military standard components are typically rated to not higher than125° C. As such, each sensor typically requires a separate continuouswire or tube to carry its output signal to a remote data acquisitionsystem, which is often located within an enclosed facility equipped witha controlled environment. The facility may be a considerable distancefrom the engine test stand, for example, 50 meters to in excess of 300meters. Routing, managing and maintaining the numerous (potentiallythousands) of data wires and tubes requires a considerable effort.Consequently, the ability to reduce the length and number of wires andtubes would be helpful and beneficial.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a hub unit adapted for use in amonitoring system adapted to monitor engine performance parameters of agas turbine engine operating on a stationary test stand, or duringon-wing flight test, or during normal aircraft operation, andparticularly a monitoring system comprising sensors mounted on theengine for sensing the engine performance parameters and generatingdigital sensor outputs.

According to a first aspect of the invention, the hub unit includes ahousing, at least one signal conditioning circuit board within thehousing and adapted to receive the analog sensor outputs from thesensors, and a control circuit board within the housing, connected tothe signal conditioning circuit board, and adapted to produce digitaldata corresponding to analog sensor outputs. The control circuit boardand the signal conditioning circuit board each comprise electricalcircuit components that define an analog signal processing path and haveaccuracy and precision characteristics that drift in response tocomponent aging and to changes in the temperature to which the hub unitis subjected. The hub unit further includes means for performing acontinuous calibration scheme by periodically applying a referencevoltage and a zero voltage to the signal conditioning circuit board todetermine and remove errors in the analog signal processing pathresulting from the drifts of the electrical circuit components of thecontrol circuit board and the signal conditioning circuit board.

According to a second aspect of the invention, in addition to certainaspects recited above, the hub unit may further include means on thesignal conditioning circuit board for multiplexing a plurality of theanalog sensor outputs generated by the sensors to produce an individualmultiplexed analog output, and at least one amplifier with adjustablegain for scaling the analog sensor outputs of the individual multiplexedanalog output to produce an individual conditioned multiplexed analogoutput from which the corresponding digital data are produced. Theamplifier and the adjustable gain thereof are controlled by the controlcircuit board.

A technical effect of the invention is the ability of the hub unit tooperate at high temperatures, for example, higher temperatures thanpossible with more temperature-sensitive hardware of the typeconventionally used to process digital data. As such, data processingcan be performed at a location remote from the high temperatureenvironment being monitored. On the other hand, the hub unit andparticularly its control and signal conditioning circuit boards can bespecially adapted for high temperature operation, preferably without theuse of active cooling. Furthermore, the continuous calibration schemeremoves errors that would otherwise exist in the analog signalprocessing path as a result of the accuracy and precisioncharacteristics of the electrical circuit components of the controlcircuit board and the signal conditioning circuit board tending to driftdue to component aging and the high temperature environment of the hubunit. In accordance with the second aspect of the invention, themultiplexing capability can reduce the number of wires or cablesnecessary to transmit data to the remotely-located distributor unit.

Other aspects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a test stand for a gas turbineengine.

FIG. 2 is a block diagram representing tiered units of a monitoringsystem adapted for monitoring performance parameters of a gas turbineengine operating while mounted on a test stand, such as of the typerepresented in FIG. 1.

FIG. 3 is a block diagram representing certain components of themonitoring system of FIG. 2, including details of a processor controlboard of the monitoring system.

FIG. 4 is a block diagram representing an analog signal conditioningboard of the monitoring system of FIG. 2.

FIG. 5 schematically represents a voltage reference device for use witha collector computer of the monitoring system of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a block diagram representing various units of a monitoringsystem 10 adapted for monitoring performance parameters of a gas turbineengine while the engine is mounted and operating on a stationary teststand, for example, the test stand 100 represented in FIG. 1. The system10 can also be used to monitor an engine during on-wing flight tests, aswell as during normal aircraft operation. While the monitoring system 10is particularly well suited for monitoring a gas turbine engine, and forconvenience will be described in reference to the engine 108 and itsstand 100 represented in FIG. 1, the use of the system 10 is not limitedto such applications. Instead, the system 10 is more broadly applicableto a wide variety of situations in which there is a desire or need tomonitor performance parameters of an apparatus operating in anenvironment subjected to elevated temperatures.

As represented in FIG. 2, the system 10 is generally identified ashaving units 12, 14, 16 and 18 that are located in four environments 34,36, 38 and 40 with respect to the gas turbine engine 108. The first unit12 comprises an array of sensors 20 appropriately located in and aroundthe engine 108 to monitor performance parameters of the engine 108 forthe purpose of assessing the performance of the engine. Any number ofsensors 20 may be employed by the system 10, and the sensors 20 may beof various types, for example, to monitor temperatures, pressures, flowrates, forces, rotational speeds, etc., of the engine 108, as waspreviously discussed in reference to FIG. 1. Certain types of sensors 20are typically employed in large numbers during the monitoring of engineoperation, including thermocouples, resistance temperature detectors(RTDs), and pressure transducers. Because the sensors 20 are located todirectly detect the parameters of interest, the unit (array) 12 ofsensors 20 is indicated in FIG. 2 as being located in a “hightemperature engine environment” 34, where maximum temperatures exceeding200° C. are often encountered by the system 10 and may reach as high as260° C. or more. Suitable sensors 20 for use in the system 10 arecommercially available and commonly used for monitoring gas turbineengine parameters, and therefore will not be discussed in any detailhere. The particular output signals generated by the sensors 20 willdepend on the type of sensors 20 used, though in most cases the signalswill be analog signals that must be digitized in order for their data tobe used by computer processing equipment to assess the performance ofthe engine.

The remaining primary units 14, 16 and 18 of the system 10 areidentified in FIG. 2 as located in environments 36, 38 and 40 wherelower temperatures are likely to occur. A first of these units will bereferred to as a hub unit 14, with which the sensors 20 directlycommunicate through any suitable wires, tubes, or other appropriateconnectors commonly employed with the particular type of sensors 20used. The hub unit 14 represented in FIG. 2 will typically be one of anumber of hub units 14 that may be used in the system 10, depending onthe number of sensors 20 and the number of sensors 20 each hub unit 14can manage. Similar to the environment 34 for the sensors 20, theenvironment 36 for the hub unit 14 is identified as a “high temperatureengine environment” 34, in that the hub units 14 are adapted to belocated in close proximity to the engine 108, for example, within aboutthree meters of the engine 108, such as the engine core environmentbeneath the engine cowling 114. In addition to locations directly on theengine 108 and beneath its cowling 114, other locations may includeadjacent locations on the head frame 106 or adapter 110 of the stand100, where very high temperatures are still likely to be encountered bythe system 10. For example, temperatures beneath the cowling 114 andadjacent locations on the head frame 106 or adapter 110 often exceed125° C., and can reach much higher temperatures, for example, higherthan 200° C. and potentially as high as 260° C. or more. Consequently,electronic components of the hub units 14 must be capable ofwithstanding significantly higher temperatures than is possible withconventional electronic components and even military standardcomponents.

In contrast, the environments 38 and 40 for the remaining two units 16and 18 of the system 10, referred to as a collector unit 16 and adistributor unit 18, are identified as a “near-engine environment” 38and a “low temperature environment” 40. The former is designated as suchbecause the collector unit 16 is adapted to be located in proximity tothe engine 108 but not as close to the engine core as the hub units 14.For example, the collector unit 16 may be located within the engine fancase environment or on the stand 100, such as on the head (thrust) frame106, at distances of about three to ten meters from the core of theengine 108. At these locations, temperatures will usually exceed 55° C.,but are significantly less than 260° C. and typically less than 125° C.Consequently, electronic components of the collector unit 16 musttypically be capable of withstanding high temperatures, though not ashigh as the hub units 14. In some situations, military standardcomponents rated up to 125° C. may be used, and possibly conventionalelectronic components rated up to 85° C.

On the other hand, the low temperature environment 40 of the distributorunit 18 permits the use of conventional electronic components rated atno more than 85° C. The environment 40 is designated as “lowtemperature” in that the distributor unit 18 can be and preferably islocated in a controlled-temperature environment, for example, anenclosed facility that is near the test stand 100 and is stabilized withair-conditioning to maintain a temperature of less than 55° C. Foron-wing engine operation, the environment 40 may be within the aircraft.The distributor unit 18 preferably has the most processing power of thesystem 10, and therefore will typically comprise one or more computerservers, personal computers, and/or other processing equipment adaptedfor data processing, collectively represented by a distributor computer42 in FIG. 2. In addition to a real-time calibration functionalitydiscussed below, the distributor computer 42 may also provide thecapability of engineering unit conversion, system configuration, anddatabase functionality. Suitable equipment for the distributor computer42 are likely to be relatively sensitive to temperature, and thereforebenefit from being housed at roughly room temperature. The lowtemperature environment 40 is typically remotely located from the enginetest stand 100, for example, in excess of fifty meters.

FIG. 2 schematically represents the hub unit 14 as comprising aprocessor control board 22 and one or more analog signal conditioningboards 24. These boards 22 and 24 are preferably enclosed in a housing44, schematically represented in FIG. 2 as completely surrounding andenclosing the boards 22 and 24. The processor control board 22 andanalog signal conditioning boards 24 operate together to convert theanalog output signals of the sensors 20 to digital data that can beprocessed by the distributor unit 18. According to certain preferredaspects of the invention, the processor control board 22 and analogsignal conditioning boards 24 also combine to perform additionalprocesses to ensure the integrity of the analog output signals receivedfrom the sensors 20 prior to their analog-digital conversion. As will beexplained in more detail below, one such additional process is toprovide a continuous calibration feature that detects any drift in theaccuracy and precision characteristics of the electronic components ofthe analog signal conditioning boards 24 and the processor control board22 that can result from component aging and variations in temperature,such as the extreme temperature changes to which the hub unit 14 issubjected. The calibration feature produces calibration data that can beused by the distributor computer 42 to perform real-time corrections ofthe digital data acquired from the hub unit 14 via the collector unit16, and more particularly a collector computer 26 of the unit 16.Another preferred process is to multiplex the analog output signals ofmultiple sensors 20 into multiplexed analog outputs, thereby reducingthe number of connections required to transmit the digital data to thecollector unit 16 over, for example, a serial data connector such as anRS-485 serial communications cable. Still another preferred process isto interleave the multiplexed analog outputs of one group (bank) ofsensors 20 with the multiplexed analog outputs of other banks of sensors20, so that the individual analog output signals of the multiplexedanalog outputs more quickly “settle” between the sets of outputs. Theseand other aspects of the hub unit 14 will be discussed in further detailbelow.

The collector unit 16 is schematically represented in FIG. 2 ascomprising the collector computer 26, a power supply 28, and atemperature-controlled environment 30 that contains a system voltagereference device 32, as will be explained in more detail below. Theprimary function of the power supply 28 is to supply power to theelectronic components of the system 10, including the sensors 20 (as maybe required) and the electronic components housed in the hub unit 14. Apreferred power supply 28 is a dual topology design with a switchingregulator front end and linear regulator back end. The power supply 28may be configured to generate multiple independently-regulated voltagesfor each hub unit 14 to increase system fault tolerance and decreasenoise coupling. The collector computer 26 receives the digital data fromthe hub unit 14, as well as any additional hub units 14 contained in thesystem 10, prior to forwarding the digital data to the distributorcomputer 42 of the distributor unit 18. The collector computer 26 ispreferably configured to have a logging capability for synchronizing theflow of the digital data to the distributor computer 42, for example,utilizing inter-range instrumentation group (IRIG) time codes or aNetwork Time Protocol (NTP). More particularly, the collector computer26 preferably operates as an intelligent switch for the incoming digitaldata from multiple hub units 14 by accurately time stamping multiplestreams of digital data coming from the hub units 14, packing the datainto frames, and then transmitting the data to the distributor computer42, for example, over a fiber-based Ethernet connection. Suitablecomponents for time stamping multiple data streams and packing data intoframes are well known in the art, and therefore will not be discussed inany detail here. The use of a fiber optic cable for the data connectionbetween the collector computer 26 and the distributor computer 42 ispreferred for the purpose of reducing the susceptibility of thetransmission to lightening, which is desirable since the transmissioncable will typically be exposed to an outdoor environment as a result ofbeing routed between the test stand 100 and the remote facility housingthe distributor unit 18. The collector computer 26, power supply 28 andcontrolled environment 30 may all be enclosed within a suitableprotective housing (not shown) that protects these components fromdirect exposure to the elements.

Notably, because of multiplexing at the level of the hub units 14 andsynchronization at the level of the collector unit 16, the digital datacan be supplied to the distributor unit 18 over a single Ethernetconnection, which is in stark contrast to the typical thousands ofcables and tubes previously required to transmit sensor output to aremote data acquisition system of the prior art.

FIG. 3 is a block diagram representing the processor control board 22,some of its components, and its connection to the analog signalconditioning boards 24 and the collector unit 16. The processor controlboard 22 is represented as being equipped with a microprocessor 46adapted to run from a program stored in ROM (read-only memory) 48, suchas an EEPROM (electrically-erasable programmable read only memory), anduses RAM (random access memory) 50 to store the digital data generatedfrom the sensors 20, as well as any variables used in calculationsperformed by the control board 22. The microprocessor 46 preferablyperforms a gain setting function associated with the signal conditioningboards 24 (discussed below), controls/selects which individual or blocksof signal channels of the sensors 20 are read, the timing of the dataacquisition, error sensing, analog-to-digital conversion, execution ofany built-in test (BIT) modes, sensor adaptation (based on the types ofsensors 20), and the collection, formatting and transfer of the digitaldata to the collector computer 26. As indicated in FIG. 3, theinput/output (I/O) functions of the board 22 are preferably directed inthe form of memory mapped I/O operations. As also seen in FIG. 3, theprocessor control board 22 also transmits zero and full-scale controloutputs to the analog signal conditioning boards 24, as well as directlycommunicates with the collector computer 26. As will be discussed inreference to the conditioning boards 24 and FIG. 4, the zero andfull-scale control outputs transmitted by the control board 22 are partof a continuous calibration scheme that periodically applies a zerovoltage and reference voltage to detect and compensate for any drift inthe accuracy and precision characteristics of the electronic componentsof the conditioning boards 24 resulting from variations in temperatureand component aging.

As previously noted, the hub unit 14 is intended to operate attemperatures greater than 125° C., and preferably as high as at least200° C. In preferred embodiments, the microprocessor 46, ROM 48, RAM 50and passive components mounted to the control board 22 are capable ofoperating at temperatures above 200° C. To achieve this capability, themicroprocessor 46, ROM 48 and RAM 50 are preferably implemented withsilicon-on-insulator (SOI) substrates and processing technology. Asknown in the art, SOI substrates typically comprise a thin epitaxiallayer on an insulator. The substrate is typically formed by oxidizingone or both bonding surfaces of a pair of semiconductor (e.g., silicon)wafers prior to bonding the wafers. Most typically, a single silicondioxide layer is grown on an epitaxial layer formed on a silicon wafer.After bonding the wafers, all but the insulator and epitaxial layer (andoptionally the silicon layer of the second wafer) are etched away, suchthat the silicon dioxide layer forms an insulator that electricallyisolates the epitaxial layer. A commercial example of a solid-statemicroprocessor implemented on an SOI substrate using SOI processingtechnology is the HT83C51 microprocessor commercially available fromHoneywell. Commercial examples of RAM components implemented on SOIsubstrates include the HT6256 256 Kbit SRAM component available fromHoneywell, and commercial examples of ROM components implemented on SOIsubstrates include ROM components from Twilight Technology Inc.

The substrate on which the electronic components of the processorcontrol board 22 are mounted is also preferably capable of withstandingtemperatures of at least 260° C. A preferred high-temperature substratematerial is commercially available from Rogers Corporation under thename RO4003C, which is a glass-reinforced hydrocarbon/ceramic laminate.Furthermore, the components are preferably attached with high meltingpoint solders, a notable but nonlimiting example of which is92.5Pb-5Sn-2.5Ag, which has a melting range of about 287 to about 296°C. To reduce thermal stresses resulting from thermal expansion andcontraction of the board, the microprocessor 46, ROM 48, RAM 50 andother components on the board 22 are preferably through-hole componentshaving one or more metal leads (sticks) that are inserted intothrough-holes (typically plated through-holes) in the substrate and thensoldered to the substrate. Other approaches to reducing thermal stressesinclude the use of high-temperature, thermally-conductive pottingmaterials to minimize thermal gradients, increase thermal time constantsand damp vibrations, and limiting the number of metallized vias that aresusceptible to breaking due to board delamination andexpansion/contraction. Notably, the metal leads of the through-holecomponents are believed to promote the structural integrity of the viasin which they are placed.

With the above-noted high temperature capabilities, the control board 22can be contained within the hub unit housing 44, preferably without theneed for an active cooling system dedicated to maintaining thetemperature of the board 22 below 125° C. as would be required byconventional electronics. The term “active cooling” is used herein tomean cooling systems that are specifically designed to transfer heatfrom the board 22 and out of the hub unit housing 44 by conduction,convection, and/or radiation.

FIG. 4 is a block diagram representing two analog signal conditioningboards 24 and their connection to the processor control board 22 of FIG.3. The analog signal conditioning boards 24 are combined with theprocessor control board 22 within the housing 44 of the hub unit 14, andas such are also required to operate at high temperatures in the harshenvironment of the engine 108. The high temperature operation of the hubunit 14 and its conditioning boards 24 enables the sensors 20, includingthermocouples, RTDs, and pressure transducers, to be terminated directlyon the engine 108 and their outputs conditioned prior to the A/D(analog-to-digital) conversion performed by the processor control board22. Furthermore, the hardware of the conditioning boards 24 preferablyincorporates the previously-noted continuous calibration, multiplexingand interleaving features.

As note above, the continuous calibration scheme performed on theconditioning boards 24 produces calibration data that can be used by thedistributor computer 42 to perform real-time corrections of the digitaldata acquired from the hub unit 14. The continuous calibration schemepreferably compensates for all passive and active components on theconditioning boards 24 and processor control board 22 that maysignificantly affect signal accuracy. The need for a continuouscalibration feature arises because, at the system level, discretecomponents are not currently available that do not exhibit drift overthe foreseeable operating range of the hub unit 14, for example, about−55° C. to above 200° C. In preferred embodiments of the invention, thecontinuous calibration scheme provides for zero and full-scale data tobe continuously collected, while any drifting of the acquired data overtime and temperature is automatically compensated.

The continuous calibration feature relies in part on the system voltagereference device 32, represented in FIG. 2 as located in the controlledenvironment 30 of the collector unit 16 remote from the hub unit 14.Though a location with the collector unit 16 is believed to bepreferred, it is foreseeable that other locations could be foundsuitable for system voltage reference device 32. The controlledenvironment 30 is schematically represented in greater detail in FIG. 5as comprising the voltage reference device 32 enclosed within a housing52, which further contains a heating element 54, copper plate 56 andthermal RTV potting material 58 that achieve uniform heating of thevoltage reference device 32. The temperature of the reference device 32can be regulated to any suitable level, for example, about 55° C. toabout 125° C. The reference device 32 generates highly-precise zero andfull-scale reference voltages, which are then transmitted over dedicateddifferential links to the conditioning boards 24.

Temperature-induced drifting in the accuracy and precision of theelectrical circuit components of the conditioning boards 24 are capturedand recorded along with the analog output signals of the sensors 20during A/D conversion. During each cycle in which analog output signalsare read from the sensors 20, the processor control board 22 causes thehighly-precise zero volt and reference voltage signals of the referencedevice 32 to be transmitted through all analog signal processing paths(channels) defined by the electronic components of each conditioningboard 24. The zero volt and reference voltage signals are then used tocorrect the digitalized sensor data, in that any change in the outputvoltage from the previous calibration reading is attributed toboard-level component drift and transmitted as calibration data to thedistributor computer 42, which digitally corrects the digitalized sensordata before further use of the data. In practice, the zero andfull-scale reference signals may be applied several times per second.Accuracies over time, temperature and distance on the order of having anaccuracy on the order of about +/−20 ppm (parts per million) and lesshave been achieved in the analog signal processing path with thecontinuous calibration feature described above.

As part of the calibration scheme, the conditioning boards 24 alsoprovide for multiplexing of multiple signal channels from the sensors20, enabling each conditioning board 24 to condition multiple sensorsignals through a fewer number of circuit paths, for example, two asrepresented in FIG. 4. Signals from multiple sensors 20 are representedin FIG. 4 as passing through multiplexors 60 to generate multiplexedanalog outputs, thereby reducing the number of connections required totransmit the digital data to the collector unit 16 over a serial dataconnector. Within each circuit path, the multiplexed analog outputs areconditioned with an instrumentation operational amplifier 62. Eachamplifier 62 is represented in FIG. 4 as incorporating active gainchanges 64 controlled by the processor control board 22, which enableseach conditioning board 24 to be used to scale many different sensortypes with different voltage outputs to a set output voltage prior toA/D conversion.

As further evidenced from FIG. 4, switches 68 can be used to interleavethe multiplexed analog outputs of one bank (group) of sensors 20 alongone circuit path on the board 24 with the multiplexed analog outputs ofanother bank of sensors 20 on another circuit path of the same board 24to increase system throughput. As one series of multiplexed analogoutputs from one bank of sensors 20 is output to the A/D converter ofthe processor control board 22, sensor outputs on other banks of sensors20 are at various stages of settling. Once the series of multiplexedanalog outputs from the first bank of sensors 20 has been read by theA/D converter, the next bank can be chosen while signals of the firstbank begin to settle on a different sensor 20. This feature allows ahigher system level throughput to be implemented with slower, buthigher-temperature capable circuit components of the processor controlboard 22 and conditioning boards 24.

The conditioning board 24 depicted in FIG. 4 is further represented asincorporating dynamic, dual-time constant filtering 66 on the amplifieroutputs of each operational amplifier 62 and controlled by the processorcontrol board 22. This feature further enables rapid settling whenswitching between the circuit paths over which the multiplexed analogoutputs are transmitted, while still providing a high level of low passfiltering to reduce electrical noise of a sensor output signal presentin the engine test environment. Dynamic filtering can be achieved by,for example, removing a resistor from an RC circuit, allowing a rapidoutput change from one channel voltage to another, then switching theresistor back into the circuit to minimize sensor noise and ripple,improving the analog data quality presented to the A/D converter (ADC).

Notably, each conditioning board 24 is preferably able to accommodateboth positive and negative input voltages, for example, in the eventthat the sensors 20 include thermocouples and pressure transducers thatcan output negative voltages. Additionally, because the conditioningboards 24 are located in the high temperature environment of the hubunit 14, “cold junction” compensation conventionally performed onthermocouples board can be “hot junction” compensation sincethermocouples among the sensors 20 may be at a lower temperature thanthe thermocouple wire-to-reference junction measured by the conditioningboard 24. For this reason, the instrumentation operational amplifiers 62are preferably capable of differential voltages and scalesthese±voltages to a positive-only voltage range necessary for A/Dconversion.

As with the processor control board 22, at least some of the circuitcomponents of the analog signal conditioning boards 24 are preferablyimplemented with SOI technology to allow operation of the boards 24 attemperatures of at least 200° C., enabling the entire hub unit 14 tooperate at such elevated temperatures. As a result, the hub unit 14 andits control and conditioning boards 22 and 24 overcome prior limitationsof data acquisition systems that have necessitated that each individualsensor output must be transmitted by wire or tube to a remote location aconsiderable distance from an engine under test. Such restrictions haveresulted in long wires and tubes routed from engines to the dataacquisition systems, incurring additional expense, introducingadditional sources of error, and necessitating a considerable amount ofman-hours to install and debug. In contrast, the hub unit 14 can beplaced directly on the head frame 106, its adapter 110, or even directlyon the engine 108, for example, under the cowling 114, resulting in arelatively short distance (for example, less than three meters) betweenthe sensors 20 and their terminations on the hub unit 14.

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. For example, the physical configuration of the units12, 14, 16 and 18 and the components could differ from that shown, andmaterials and processes other than those noted could be used. Therefore,the scope of the invention is to be limited only by the followingclaims.

The invention claimed is:
 1. A hub unit adapted for use in a monitoringsystem for monitoring engine performance parameters of a gas turbineengine while operating, the monitoring system comprising sensors mountedon the engine for sensing the engine performance parameters andgenerating analog sensor outputs, the hub unit comprising: a housing; acontrol circuit board and at least one signal conditioning circuit boardwithin the housing, the signal conditioning circuit board being adaptedto receive the analog sensor outputs from the sensors, the controlcircuit board being connected to the signal conditioning circuit boardand adapted to produce digital data corresponding to the analog sensoroutputs, the control circuit board and the signal conditioning circuitboard comprising electrical circuit components that define an analogsignal processing path and have accuracy and precision characteristicsthat drift in response to component aging and to changes in thetemperature to which the hub unit is subjected; and means for performinga continuous calibration scheme by periodically applying a referencevoltage and a zero voltage to the signal conditioning circuit board todetermine and remove errors in the analog signal processing pathresulting from the drifts of the electrical circuit components of thecontrol circuit board and the signal conditioning circuit board.
 2. Thehub unit according to claim 1, wherein the hub unit is mounted on theengine or on a test stand supporting the engine.
 3. The hub unitaccording to claim 1, wherein the hub unit is subjected to a temperaturein a range of in excess of 125° C. up to at least 200° C.
 4. The hubunit according to claim 1, wherein the means for performing thecontinuous calibration scheme periodically applies the reference voltageand the zero voltage to the signal conditioning circuit board at afrequency of more than once per second.
 5. The hub unit according toclaim 1, wherein at least some of the electrical circuit components ofthe control circuit board and the signal conditioning circuit board areimplemented on silicon-on-insulator substrates.
 6. The hub unitaccording to claim 1, wherein the electrical circuit components have amaximum operating temperature of greater than 125° C.
 7. The hub unitaccording to claim 1, wherein the hub unit lacks means for activelycooling the hub unit.
 8. The hub unit according to claim 1, wherein thereference voltage is a full scale voltage of the signal conditioningcircuit board.
 9. The hub unit according to claim 1, wherein the removalof errors in the analog signal processing path with the means forperforming the continuous calibration scheme results in the analogsignal processing path having an accuracy of at least +/−20 ppm.
 10. Thehub unit according to claim 1, wherein the means for performing thecontinuous calibration scheme determines the errors in the analog signalprocessing path by comparing outputs generated by applying the referencevoltage and the zero voltage to values of the reference voltage and thezero voltage, and then correcting subsequent digital data based on thecomparison.
 11. The hub unit according to claim 10, wherein the meansfor performing the continuous calibration scheme completes thecontinuous calibration scheme each time analog sensor outputs areacquired.
 12. The hub unit according to claim 1, further comprising:means on the signal conditioning circuit board for multiplexing aplurality of the analog sensor outputs generated by the sensors toproduce an individual multiplexed analog output; and at least oneamplifier with adjustable gain for scaling the analog sensor outputs ofthe individual multiplexed analog output to produce an individualconditioned multiplexed analog output from which the correspondingdigital data are produced, the amplifier and the adjustable gain thereofbeing controlled by the control circuit board.
 13. The hub unitaccording to claim 12, wherein the individual multiplexed analog outputproduced by the multiplexing means is one of a plurality of individualmultiplexed analog outputs produced by the multiplexing means and scaledby the at least one amplifier to produce a plurality of individualconditioned multiplexed analog outputs, and each of the individualmultiplexed analog outputs is produced from a corresponding set of theanalog sensor outputs generated by the sensors, wherein the signalconditioning circuit board further comprises means for interleaving theindividual conditioned multiplexed analog outputs to reduce settlingtime between the sets of the analog sensor outputs and improvethroughput to the control circuit board.
 14. The hub unit according toclaim 13, wherein the interleaving means comprises a dynamic filter thatswitches passive RC components into and out of a circuit path containingthe multiplexing means, the amplifier, and the interleaving means toincrease throughput by reducing the settling time.
 15. A hub unitadapted for use in a monitoring system for monitoring engine performanceparameters of a gas turbine engine operating on a stationary test stand,the monitoring system comprising sensors mounted on the engine forsensing the engine performance parameters and generating analog sensoroutputs, the hub unit comprising: a housing; a control circuit board andat least one signal conditioning circuit board within the housing, thesignal conditioning circuit board being adapted to receive the analogsensor outputs from the sensors, the control circuit board beingconnected to the signal conditioning circuit board and adapted toproduce digital data corresponding to the analog sensor outputs, thecontrol circuit board and the signal conditioning circuit boardcomprising electrical circuit components that define an analog signalprocessing path and have accuracy and precision characteristics thatdrift in response to component aging and to changes in the temperatureto which the hub unit is subjected; means for performing a continuouscalibration scheme by periodically applying a reference voltage and azero voltage to the signal conditioning circuit board to determine andremove errors in the analog signal processing path resulting from thedrifts of the electrical circuit components of the control circuit boardand the signal conditioning circuit board; means on the signalconditioning circuit board for multiplexing a plurality of the analogsensor outputs generated by the sensors to produce an individualmultiplexed analog output; and at least one amplifier with adjustablegain for scaling the analog sensor outputs of the individual multiplexedanalog output to produce an individual conditioned multiplexed analogoutput from which the corresponding digital data are produced, theamplifier and the adjustable gain thereof being controlled by thecontrol circuit board.
 16. The hub unit according to claim 15, whereinthe hub unit is mounted on the engine or on the test stand.
 17. The hubunit according to claim 15, wherein the hub unit is subjected to atemperature in a range of in excess of 125° C. up to at least 200° C.